1. Technical Field
The present invention relates to an electrophotographic photoreceptor, a process cartridge, an image forming apparatus and an image forming method to be employed in the electrophotographic image forming system, more in detail, relates to an electrophotographic photoreceptor, the process cartridge, the image the forming apparatus and the image forming method to be employed in the electrophotographic image forming system applied in the field of copy machine and printer.
2. Related Art
Organic photoreceptors have many merits such as that the materials can be selected from wide range, excellent in the suitability to the environmental and the production cost is low compared with inorganic photoreceptors such as a selenium photoreceptor and an amorphous silicone photoreceptor. Therefore, the organic photoreceptor recently becomes as the main stream of the electrophotographic photoreceptor in place of the inorganic photoreceptors.
In the image forming system according to the Carlson method, the electrophotographic photoreceptor is charged and a static latent image is formed thereon, and the a toner image is formed and the toner image is transferred onto image receiving paper and fixed to form a final image.
Besides, a digital image forming system employing a LED or a laser as the light source for imagewise exposing is rapidly spread in the image forming method of the recent electrophotographic system. For example, a technique for making an high quality electrophotographic image is disclosed, in which the imagewise exposure is given by a laser beam having a small spot area so as to form a high precise latent image with high dot latent image density, and then the latent image is developed by a fine particle toner (Japanese Patent publication Open to Public Inspection, hereinafter referred to as Japanese Patent O.P.I. Publication, No. 2001-255685).
Recently, miniaturization and high speed driving of the electrophotographic apparatus such as the digital copying machine and the printer are progressed and the photoreceptor is required at the same time to make higher the sensitivity corresponding to the high speed driving and to improve the anti-wearing ability for prolonging of the life thereof.
However, problems of the degradation of the charging ability and the stability of the sensitivity are caused when a charge generation material and a charge transfer material suitable for such the high sensitivity and the high speed driving are used. Namely, problems that the charged potential tends to be lowered and the remaining potential tends to be increase under a high temperature and high humidity condition or a low temperature and low humidity condition are caused.
The degradation in the charged potential and the stability of sensitivity causes lowering the difference between the potential at the unexposed area (VH) and that at the exposed area (VL) so as to lower the image density, and the difference between potential at the unexposed area VH and the direct current bias potential (VDC) applied to the photoreceptor and the developing sleeve is also lowered so as to tend to occur a image fault such as black spots.
A technique has been developed in which an intermediate layer is used in the organic photoreceptor to stabilize the charged potential and to solve the problems of the image defect such as black spots. For example, an organic photoreceptor is disclosed in which the intermediate layer is provided between the electroconductive substrate and the photosensitive layer; the intermediate layer is composed of a resin in which titanium oxide particles are dispersed. Furthermore, an intermediate layer containing surface treated titanium oxide is known. For example, organic photoreceptors having an intermediate layer containing titanium oxide surface-treated by iron oxide or tungsten oxide (Japanese Patent O.P.I. Publication No. 4-303846), that containing titanium oxide surface treated by an amino acid-containing coupling agent (Japanese Patent O.P.I. Publication No. 9-96916)., that containing titanium oxide surface-treated by an organic silicon compound (Japanese Patent O.P.I. Publication No. 9-258469), that containing titanium oxide surface-treated by methylhydrogenpolysiloxane (Japanese Patent O.P.I. Publication No. 8-328283) or that containing dendrite-shaped titanium oxide surface-treated by a metal oxide or an organic compound(Japanese Patent O.P.I. Publication No. 11-344826) have been proposed.
Under the serious condition such as high temperature and high humidity, however, the prevention of occurrence of the black spots is insufficient and problems such as that the raising in the remaining potential and in the potential at the exposed area caused accompanied with the repeating use so that sufficient image density cannot be obtained even when such the techniques are applied.
Moreover, it has been proposed that the crystal structure is further exactly controlled for improving the occurrence of the black spots and the raising in the remaining potential and in the potential at the exposed area accompanied with repeating use. For example, an intermediate layer containing anatase type titanium oxide pigment, hereinafter referred to as anatase type titanium oxide or anatase type titanium oxide particle, has been proposed ((Japanese Patent O.P.I. Publication No. 11-327188). The anatase type titanium oxide is lower in the volume resistance than the rutile type titanium oxide. Consequently the thickness of the intermediate layer can be thicker and the irregularity of the electroconductive substrate surface can be concealed by such the thick layer so as to easily prevent the injection of the charge from the electroconductive substrate. Besides, The dark attenuation of the charged potential is increased and fogging by the reversal development tends to increase. Such the contrary problems cannot be sufficiently solved yet by the disclosure of the patent publications.
For example, the method is well known in which the intermediate layer is formed by dispersing titanium oxide particles in polyamide resin. The polyamide resin employed in such the case, usually a copolymerized amide resin mainly constituted by a chemical structure having a small number of carbon atoms such as 6-nylon and a tri-methylized polyamide resin, has high water absorbability. The intermediate layer using such the polyamide resin tends to be high in the environment dependency. As a result of that, the charging property is easily varied under the high temperature and humidity condition and the black spots easily occur.
It is prospected that the copolymerized polyamide resin constituted by a constituting unit having large number of carbon atom between the amide bonds such as 12-nylon is suitable material for producing the photoreceptor having low environmental dependency since such the resin has low water absorbability. However, such the polyamide resin does not suit for producing the photoreceptor since the resin is insoluble in a usual organic solvent. Though examples in which the solubility of the resin is increased by methoxylization are disclosed ((Japanese Patent O.P.I. Publication Nos. 5-72787 and 6-186767), the occurrence of the black spots and the environmental memory is difficultly prevented since the water absorption of the resin is considerably increased by the methoxymethylization.
Though examples in which a compound having a Si-containing glassy network subunit, a flexible organic subunit and photo-electrical subunit (Japanese Patent O.P.I. Publication No. 11-316468), and a resin layer containing an organic polymer, a siloxane condensation product and an anti-oxidation component is applied as outer most layer (Japanese Patent O.P.I. Publication No. 2002-236382) have been reported, the application of these resins in the intermediate layer for solving the foregoing problems are not reported.
Hitherto a corona discharging device is typically employed as the charging means. However, the corona discharging device causes problems that the degradation of the organic photoreceptor, hereinafter referred to as photoreceptor, and bad influence on the human body are caused since high voltage should be applied to the corona discharging device and large amounts of ionized oxygen, ozone, moisture and a nitrogen oxide compound are generated.
Recently, application of contact charging method without the use of the corona discharging device is investigated. In concrete, a magnetic brush or an electroconductive roller as the charging means, to which voltage is applied, is contacted to the photoreceptor as the material to be charged to charge the photoreceptor surface at the designated potential. The applying voltage can be lowered and the generation amount of ozone can be reduced by the use of such contacting charge method compared with the non-contact charging method using the corona discharging device.
In the contact charging method, a charging member having a resistance of about from 102 to 1010 Ω·cm is contacted with pressure to the photoreceptor while applying direct current or direct current overlapped with alternative current voltage to the charging member for providing the charge. In this charging method, the charging is started by applying voltage higher than a certain threshold value since the discharge from the charging member to the subjective member is progressed according to Paschen's law. In the contact charging method, the applying voltage is lower that in the corona charging method and the generation amount of ozone and nitrogen oxide compound is reduced.
However, the repeatedly charging to the electrophotographic photoreceptor by the contacting of the charging roller causes cracks and contamination on the photoreceptor surface. As a result of that, the charge is concentrated to the cracks and the contaminated portion so that the image defects such as dielectric breakdown and black spot are tend to occur and the spreading of the image also occurs. Such the problems are easily posed in under the serious conditions such as high temperature and high humidity and low temperature and low humidity.
It is proposed to prevent the occurrence of the dielectric breakdown and the black spot that the surface of the electroconductive substrate is anodized for raising the resistivity of the photoreceptor against leak of charge and so as to prevent the charge leak from the electroconductive substrate even when the cracks and the contamination are formed on the photoreceptor surface (Japanese Patent O.P.I. Publication No. 5-080567).
A new charging system in which the charge is directly injected into the photoreceptor has been proposed (Japanese Patent O.P.I. Publication No. 6-3921). In this method, a charge injection layer is provided on the photoreceptor surface and the charge is injected by contacting with a contacting electroconductive member such as a charging roller, a charging brush and a charging magnetic brush to which voltage is applied. The charging almost 1 by 1 with respect to the charged voltage can be performed by this charging method, with almost no discharging phenomenon. Accordingly, the method is an excellent charging method since the generation amount of ozone and NOx is considerably small and the necessary electric power is low.
An electrophotographic photoreceptor for the injection charging has be known, in which the charge injection layer is provided on the surface (mainly on the charge transfer layer) of the photoreceptor. Namely, an electrophotographic photoreceptor having the charge injection layer containing a binder resin and an electroconductive fine particle or a charge transfer material as the outermost layer is proposed (Japanese Patent O.P.I. Publication No. 2002-31911). However, when the charging through the charge injection is repeatedly performed by the directly contacting with the charging roller, cracks and contamination are formed on the charge injection layer and the charge is concentrated to the portion of the cracks and the contamination so as to tend to result the image defect such as the dielectric breakdown and the black spots and the spreading of the image is also tend to occur. These problems particularly tend to be posed under the serious conditions such as high temperature and humidity and the low temperature and humidity.
In the electrophotographic photoreceptor employing the anodized aluminum substrate, however, the properties of anodized layer is varied depending on the slight variation of the conditions of anodized treatment and that of the storage thereafter so that the above-described charge leak preventing effect is difficultly obtained, furthermore, it is observed that the interface of the anodized layer and the light-sensitive layer tends to be a charge trap site and the remaining potential is gradually increased accompanied with the prolongation of the using period.